Multilayer retaining ring for chemical mechanical polishing

ABSTRACT

A carrier head for a chemical mechanical polishing apparatus includes a retaining ring having a flexible lower portion and a rigid upper portion. A shim can be inserted between the retaining ring and a base of the carrier head to improve the retaining ring lifetime. A seal may be inserted between the retaining ring and a flexible membrane to seal the chamber between the flexible membrane and the base.

BACKGROUND

The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for a chemical mechanical polishing apparatus.

Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.

Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a “standard” or a fixed-abrasive pad. A standard polishing pad has durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles, if a standard pad is used, is supplied to the surface of the polishing pad.

The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.

A reoccurring problem in CMP is the so-called “edge-effect”, i.e., the tendency of the edge of the substrate to be polished at a different rate than the center of the substrate. The edge effect typically results in over-polishing (the removal of too much material from the substrate) at the substrate perimeter, e.g., the outermost five to ten millimeters of a 200 mm wafer. Over-polishing reduces the overall flatness of the substrate, causing the edge of the substrate to be unsuitable for integrated circuit fabrication and decreasing the process yield.

SUMMARY

In one aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus. The carrier head has a carrier base, a retaining ring, a flexible membrane, and a shim. The retaining ring includes a lower portion having a bottom surface for contacting a polishing pad during polishing and made of a first material, and an upper portion made of a second material which is more rigid than the first material. The flexible membrane has a central portion that provides a substrate mounting surface and an outer portion located between the base and the upper portion of the retaining ring. The shim is located between the carrier base and the upper portion of the retaining ring.

Implementations of the invention may include one or more of the following features. A second passage in the retaining ring may align with a first passage in the carrier head. A bolt or screw may extend through the first and second passages to secure the retaining ring to the carrier base. The bolt or screw may extend through an aperture in the shim into the second passage in the retaining ring. The second passage may not extend into the lower portion of the retaining ring. The shim may be composed of stainless steel. The first material may be tube PPS and the second material may be stainless steel. A seal may be clamped between the outer portion of the flexible membrane and the upper portion of the retaining ring to seal a chamber between the carrier base and the flexible membrane.

In another aspect, the invention is directed to a retaining ring for a carrier head having a mounting surface for a substrate. The retaining ring has a generally annular lower portion having a bottom surface for contacting a polishing pad during polishing and made of a first material which is inert in a chemical mechanical polishing process, and a generally annular upper portion joined to the lower portion and made of a second material which is more rigid than the first material. The first material is tube PPS and the second material is a metal.

In another aspect, the invention is directed to a retaining ring for a carrier head having a mounting surface for a substrate. The retaining ring has generally annular lower portion and a generally annular upper portion joined to the lower portion. The lower portion has a bottom surface for contacting a polishing pad during polishing and a plurality of channels in the lower surface of the retaining ring to carry slurry inwardly. The channels have a depth of at least about 0.14 inches. The lower portion is made of a first material which is inert in a chemical mechanical polishing process. The upper portion is made of a second material which is more rigid than the first material.

In another aspect, the invention is directed to a method of using a retaining ring. In the method, the retaining ring is secured to a base in a carrier head, and an outer edge of a flexible membrane is clamped between the retaining ring and the base. A first plurality of substrates are polished with the carrier head with a lower surface of the retaining ring contacting a polishing surface. The retaining ring is removed from the carrier head after the lower surface of the ring has been worn by a first amount. The retaining ring is resecured to the carrier head with a first shim between the base and the retaining ring, and a second plurality of substrates are polished with the carrier head.

Implementations of the invention may include one or more of the following features. A first seal may be secured between the flexible membrane and the retaining ring before polishing the second plurality of substrates. The retaining ring and first shim may be removed from the carrier head after the lower surface of the retaining ring has been worn by a second amount, the retaining ring may be resecured to the carrier head with a second shim between the base and the retaining ring, and a third plurality of substrates may be polished with the carrier head. A second seal may be secured between the flexible membrane and the retaining ring before polishing the third plurality of substrates.

In another aspect, the invention is directed to a kit with a retaining ring with an upper portion, a shim to be inserted between a base of a carrier head and the upper portion of the retaining ring, and an annular seal to be inserted between a flexible membrane and the retaining ring. The retaining ring includes a lower portion having a bottom surface for contacting a polishing pad during polishing and made of a first material, and an upper portion made of a second material which is more rigid than the first material. A plurality of channels are formed in the bottom surface of the retaining ring for carrying slurry inwardly.

In one aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus. The carrier head has a substrate mounting surface and a retaining ring to maintain a substrate beneath the mounting surface during polishing. The retaining ring includes a lower portion having a bottom surface for contacting a polishing pad during polishing and made of a first material, and an upper portion made of a second material which is more rigid than the first material.

Advantages of implementations of the invention may include zero or more of the following. The edge effect may be reduced, and the resulting flatness and finish of the substrate may be improved. The retaining ring may have an improved lifetime.

Other advantages and features of the invention will be apparent from the following description, including the drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a chemical mechanical polishing apparatus.

FIG. 2 is a schematic cross-sectional view of a carrier head according to the present invention.

FIG. 3 is an enlarged view of the carrier head of FIG. 2 showing a retaining ring.

FIG. 4 is an enlarged view of a carrier head that has a shim and a retaining ring.

FIG. 5 is a bottom view of a portion of one implementation of the retaining ring.

DETAILED DESCRIPTION

Referring to FIG. 1, one or more substrates 10 will be polished by a chemical mechanical polishing (CMP) apparatus 20. A description of a similar CMP apparatus may be found in U.S. Pat. No. 5,738,574, the entire disclosure of which is hereby incorporated by reference.

The CMP apparatus 20 includes a lower machine base 22 with a table top 23 mounted thereon and a removable upper outer cover (not shown). Table top 23 supports a series of polishing stations 25 a, 25 b and 25 c, and a transfer station 27 for loading and unloading the substrates. Transfer station 27 may form a generally square arrangement with the three polishing stations 25 a, 25 b and 25 c.

Each polishing station 25 a-25 c includes a rotatable platen 30 on which is placed a polishing pad 32. If substrate 10 is an eight-inch (200 millimeter)or twelve-inch (300 millimeter) diameter disk, then platen 30 and polishing pad 32 will be about twenty or thirty inches in diameter, respectively. Platen 30 may be connected to a platen drive motor (not shown) located inside machine base 22. For most polishing processes, the platen drive motor rotates platen 30 at thirty to two-hundred revolutions per minute, although lower or higher rotational speeds may be used. Each polishing station 25 a-25 c may further include an associated pad conditioner apparatus 40 to maintain the abrasive condition of the polishing pad.

A slurry 50 containing a reactive agent (e.g., deionized water for oxide polishing) and a chemically-reactive catalyzer (e.g., potassium hydroxide for oxide polishing) may be supplied to the surface of polishing pad 32 by a combined slurry/rinse arm 52. If polishing pad 32 is a standard pad, slurry 50 may also include abrasive particles (e.g., silicon dioxide for oxide polishing). Typically, sufficient slurry is provided to cover and wet the entire polishing pad 32. Slurry/rinse arm 52 includes several spray nozzles (not shown) which provide a high pressure rinse of polishing pad 32 at the end of each polishing and conditioning cycle.

A rotatable multi-head carousel 60, including a carousel support plate 66 and a cover 68, is positioned above lower machine base 22. Carousel support plate 66 is supported by a center post 62 and rotated thereon about a carousel axis 64 by a carousel motor assembly located within machine base 22. Multi-head carousel 60 includes four carrier head systems 70 a, 70 b, 70 c, and 70 d mounted on carousel support plate 66 at equal angular intervals about carousel axis 64. Three of the carrier head systems receive and hold substrates and polish them by pressing them against the polishing pads of polishing stations 25 a-25 c. One of the carrier head systems receives a substrate from and delivers the substrate to transfer station 27. The carousel motor may orbit carrier head systems 70 a-70 d, and the substrates attached thereto, about carousel axis 64 between the polishing stations and the transfer station.

Each carrier head system 70 a-70 d includes a polishing or carrier head 100. Each carrier head 100 independently rotates about its own axis, and independently laterally oscillates in a radial slot 72 formed in carousel support plate 66. A carrier drive shaft 74 extends through slot 72 to connect a carrier head rotation motor 76 (shown by the removal of one-quarter of cover 68) to carrier head 100. There is one carrier drive shaft and motor for each head. Each motor and drive shaft may be supported on a slider (not shown) which can be linearly driven along the slot by a radial drive motor to laterally oscillate the carrier head.

During actual polishing, three of the carrier heads, e.g., those of carrier head systems 70 a-70 c, are positioned at and above respective polishing stations 25 a-25 c. Each carrier head 100 lowers a substrate into contact with a polishing pad 32. Generally, carrier head 100 holds the substrate in position against the polishing pad and distributes a force across the back surface of the substrate. The carrier head also transfers torque from the drive shaft to the substrate.

Referring to FIG. 2, carrier head 100 includes a housing 102, a base 104, a gimbal mechanism 106, a loading chamber 108, a retaining ring 110, and a substrate backing assembly 112. A description of a similar carrier head may be found in U.S. application Ser. No. 08/745,670 by Zuniga, et al., filed Nov. 8, 1996, entitled A CARRIER HEAD WITH A FLEXIBLE MEMBRANE FOR A CHEMICAL MECHANICAL POLISHING SYSTEM, and assigned to the assignee of the present invention, the entire disclosure of which is hereby incorporated by reference.

The housing 102 can be connected to drive shaft 74 to rotate therewith during polishing about an axis of rotation 107 which is substantially perpendicular to the surface of the polishing pad during polishing. The loading chamber 108 is located between housing 102 and base 104 to apply a load, i.e., a downward pressure, to base 104. The vertical position of base 104 relative to polishing pad 32 is also controlled by loading chamber 108.

The substrate backing assembly 112 includes a support structure 114, a flexure diaphragm 116 connecting support structure 114 to base 104, and a flexible member or membrane 118 connected to support structure 114. The flexible membrane 118 extends below support structure 114 to provide a mounting surface 120 for the substrate. Pressurization of a chamber 190 positioned between base 104 and substrate backing assembly 112 forces flexible membrane 118 downwardly to press the substrate against the polishing pad.

The housing 102 is generally circular in shape to correspond to the circular configuration of the substrate to be polished. A cylindrical bushing 122 may fit into a vertical bore 124 through the housing, and two passages 126 and 128 may extend through the housing for pneumatic control of the carrier head.

The base 104 is a generally ring-shaped body located beneath housing 102. The base 104 may be formed of a rigid material such as aluminum, stainless steel or fiber-reinforced plastic. A passage 130 may extend through the base, and two fixtures 132 and 134 may provide attachment points to connect a flexible tube between housing 102 and base 104 to fluidly couple passage 128 to passage 130.

An elastic and flexible membrane 140 may be attached to the lower surface of base 104 by a clamp ring 142 to define a bladder 144. Clamp ring 142 may be secured to base 104 by screws or bolts (not shown). A first pump (not shown) may be connected to bladder 144 to direct a fluid, e.g., a gas, such as air, into or out of the bladder and thereby control a downward pressure on support structure 114 and flexible membrane 118.

Gimbal mechanism 106 permits base 104 to pivot with respect to housing 102 so that the base may remain substantially parallel with the surface of the polishing pad. Gimbal mechanism 106 includes a gimbal rod 150 which fits into a passage 154 through cylindrical bushing 122 and a flexure ring 152 which is secured to base 104. Gimbal rod 150 may slide vertically along passage 154 to provide vertical motion of base 104, but it prevents any lateral motion of base 104 with respect to housing 102.

An inner edge of a rolling diaphragm 160 may be clamped to housing 102 by an inner clamp ring 162, and an outer clamp ring 164 may clamp an outer edge of rolling diaphragm 160 to base 104. Thus, rolling diaphragm 160 seals the space between housing 102 and base 104 to define loading chamber 108. Rolling diaphragm 160 may be a generally ring-shaped sixty mil thick silicone sheet. A second pump (not shown) may be fluidly connected to loading chamber 108 to control the pressure in the loading chamber and the load applied to base 104.

The support structure 114 of substrate backing assembly 112 is located below base 104. Support structure 114 includes a support plate 170, an annular lower clamp 172, and an annular upper clamp 174. Support plate 170 may be a generally disk-shaped rigid member with a plurality of apertures 176 therethrough. In addition, support plate 170 may have a downwardly-projecting lip 178 at its outer edge.

Flexure diaphragm 116 of substrate backing assembly 112 is a generally planar annular ring. An inner edge of flexure diaphragm 116 is clamped between base 104 and retaining ring 110, and an outer edge of flexure diaphragm 116 is clamped between lower clamp 172 and upper clamp 174. The flexure diaphragm 116 is flexible and elastic, although it could be rigid in the radial and tangential directions. Flexure diaphragm 116 may formed of rubber, such as neoprene, an elastomeric-coated fabric, such as NYLON or NOMEX, plastic, or a composite material, such as fiberglass.

Flexible membrane 118 is a generally circular sheet formed of a flexible and elastic material, such as chloroprene or ethylene propylene rubber. A portion of flexible membrane 118 extends around the edges of support plate 170 to be clamped between the support plate and lower clamp 172.

The sealed volume between flexible membrane 118, support structure 114, flexure diaphragm 116, base 104, and gimbal mechanism 106 defines pressurizable chamber 190. A third pump (not shown) may be fluidly connected to chamber 190 to control the pressure in the chamber and thus the downward forces of the flexible membrane on the substrate.

Retaining ring 110 may be a generally annular ring secured at the outer edge of base 104, e.g., by bolts 194 (only one is shown in the cross-sectional view of FIG. 2) that extend through aligned passages in the base 104 into the upper section of retaining ring 110. When fluid is pumped into loading chamber 108 and base 104 is pushed downwardly, retaining ring 110 is also pushed downwardly to apply a load to polishing pad 32. An inner surface 188 of retaining ring 110 defines, in conjunction with mounting surface 120 of flexible membrane 118, a substrate receiving recess 192. The retaining ring 110 prevents the substrate from escaping the substrate receiving recess.

Referring to FIG. 3, retaining ring 110 includes multiple sections, including an annular lower portion 180 having a bottom surface 182 that may contact the polishing pad, and an annular upper portion 184 connected to base 104. Lower portion 180 may be bonded to upper portion 184 with an adhesive layer 186.

The lower portion is formed of a material which is chemically inert in a CMP process. In addition, lower portion 180 should be sufficiently elastic that contact of the substrate edge against the retaining ring does not cause the substrate to chip or crack. On the other hand, lower portion 180 should not be so elastic that downward pressure on the retaining ring causes lower portion 180 to extrude into substrate receiving recess 192. Specifically, the material of the lower portion 180 may have a durometer measurement of about 80-95 on the Shore D scale. In general, the elastic modulus of the material of lower portion 180 maybe in the range of about 0.3-1.0×10⁶ psi. The lower portion should also be durable and have a low wear rate. However, it is acceptable for lower portion 180 to be gradually worn away, as this appears to prevent the substrate edge from cutting a deep grove into inner surface 188. For example, lower portion 180 may be made of a plastic, such as polyphenylene sulfide (PPS), available from DSM Engineering Plastics of Evansville, Ind., under the trade name Techtron™. Other plastics, such as DELRIN™, available from Dupont of Wilmington, Del., polyethylene terephthalate (PET), polyetheretherketone (PEEK), or polybutylene terephthalate (PBT), or a composite material such as ZYMAXX™, also available from Dupont, may be suitable. Although lower portion 180 can be formed of plate PPS, preferably lower portion 180 is formed of tube PPS.

The thickness T_(l) of lower portion 180 should be larger than the thickness T_(s) of substrate 10. Specifically, the lower portion should be thick enough that the substrate does not brush against the adhesive layer when the substrate is chucked by the carrier head. On the other hand, if the lower portion is too thick, the bottom surface of the retaining ring will be subject to deformation due to the flexible nature of the lower portion. The initial thickness of lower portion 180 may be about 200 to 400 mils (with channels having a depth of 100 to 300 mils). The lower portion may be replaced when the grooves have been worn away. Thus, the lower portion will have a thickness T_(l) between about 100 and 400 mils. If the retaining ring does not include grooves, then the lower portion may be replaced when it has a remaining thickness about equal to the substrate thickness.

The bottom surface of the lower portion 180 may be substantially flat, or as shown in FIGS. 3 and 5 it may have a plurality of channels 189 that extend from the inner surface 188 to the outer surface of the retaining ring to facilitate the transport of slurry from outside the retaining ring to the substrate. The channels 189 may have a depth T₃ of 140 mils or more.

The upper portion 184 of retaining ring 110 is formed of a rigid material, such as a metal, e.g., stainless steel, molybdenum, or aluminum, or a ceramic, e.g., alumina, or other exemplary materials. The material of the upper portion may have an elastic modulus of about 10-50×10⁶ psi, i.e., about ten to one hundred times the elastic modulus of the material of the lower portion. For example, the elastic modulus of the lower portion may be about 0.6×10⁶ psi, the elastic modulus of the upper portion may be about 30×10⁶ psi, so the ratio is about 50:1. The thickness T_(s) of upper portion 184 should be larger than the thickness T_(l) of lower portion 182. Specifically, the upper portion may have a thickness T₂ of about 300-500 mils.

The adhesive layer 186 may be a two-part slow-curing epoxy. Slow curing generally indicates that the epoxy takes on the order of several hours to several days to set. The epoxy may be Magnobond-6375TM, available from Magnolia Plastics of Chamblee, Ga. Alternately, instead of being adhesively attached, the lower portion can be attached with screws or press-fit to the upper portion.

It appears that the flatness of the bottom surface of the retaining ring is connected to the edge effect. Specifically, if the bottom surface is very flat, the edge effect is reduced. If the retaining ring is relatively flexible, it can be deformed where it is joined to the base, e.g., by bolts 194. This deformation creates a non-planar bottom surface, thereby increasing the edge effect. Although the retaining ring can be lapped or machined after installation on the carrier head, lapping tends to embed debris in the bottom surface which can damage the substrate or contaminate the CMP process, and machining is time-consuming and inconvenient. On the other hand, an entirely rigid retaining ring, such as a stainless steel ring, can cause the substrate to crack or contaminate the CMP process.

With the retaining ring of the present invention, the rigidity of upper portion 184 of retaining ring 110 increases the overall flexural rigidity of the retaining ring, e.g., by a factor of 30-40 times, as compared to a retaining ring formed entirely of a flexible material such as PPS. The increased rigidity provided by the rigid upper portion reduces or eliminates this deformation caused by the attachment of the retaining ring to the base, thereby reducing the edge effect. Furthermore, the retaining ring need not be lapped after it is secured to the carrier head. In addition, the PPS lower portion is inert in the CMP process, and is sufficiently elastic to prevent chipping or cracking of the substrate edge.

Another benefit of the increased rigidity of the retaining ring of the present invention is that it reduces the sensitivity of the polishing process to pad compressibility. Without being limited to any particular theory, one possible contribution to the edge effect, particularly for flexible retaining rings, is what may be termed “deflection” of the retaining ring. Specifically, the force of the substrate edge on the inner surface of the retaining ring at the trailing edge of the carrier head may cause the retaining ring to deflect, i.e., locally twist slightly about an axis parallel to the surface of the polishing pad. This forces the inner diameter of the retaining ring more deeply into the polishing pad, and generates increased pressure on the polishing pad and causes the polishing pad material to “flow” and be displaced toward the edge of the substrate. The displacement of the polishing pad material depend upon the elastic properties of the polishing pad. Thus, a relatively flexible retaining ring which can deflect into the pad, makes the polishing process extremely sensitive to the elastic properties of the pad material. However, the increased rigidity provided by the rigid upper portion decreases the deflection of the retaining ring, thereby reducing pad deformation, sensitivity to pad compressibility, and the edge effect.

Referring to FIG. 4, after a portion of the retaining ring has been worn away, a shim 202 can be placed between retaining ring 110 and base 104 to form part of the carrier head structure, with bolts 194 extending through apertures in shim 202. The retaining ring can be a high precision part, and once the retaining ring has been worn away by a certain amount, e.g., 40 mils, the carrier head may not function properly. For example, the lower surface of the retaining ring might not contact the polishing surface during polishing. However, shim 202 increases the distance between the base and the lower surface of the retaining ring to maintain proper function of the carrier head. In addition, an annular seal 200 can be placed between the outer edge of flexure 116 (or flexible membrane 118, if the flexure and flexible membrane are joined to form a unitary part) and retaining ring 110. Annular seal 200 prevents fluid leakage into or out chamber 190 via the gap between retaining ring 110 and base 104. The shim can be formed of a relatively rigid material, e.g., stainless steel, whereas the annular seal can be formed of a compressible material, e.g., rubber. The refurbishment process can be repeated multiple times, e.g., each time the retaining ring has been worn away by an additional 40 mils. Each time, a slightly thicker shim and annular seal would be inserted into the carrier. Thus, providing the lower surface of the retaining ring with deep grooves and a repair kit with shims and seals can significantly improve the retaining ring lifetime.

The present invention has been described in terms of a number of embodiments. The invention, however, is not limited to the embodiments depicted and described. Rather, the scope of the invention is defined by the appended claims. 

What is claimed is:
 1. A carrier head, comprising: a carrier base; a retaining ring including a lower portion having a bottom surface for contacting a polishing pad during polishing and made of a first material, and an upper portion made of a second material which is more rigid than the first material; a flexible membrane coupled to the base and having a portion that provides a substrate mounting surface; a shim located between the carrier base and the upper portion of the retaining ring; and a seal coupled to the flexible membrane to seal a chamber between the carrier base and the flexible membrane.
 2. The carrier head of claim 1, further comprising a first passage in the carrier base, a second passage in the retaining ring aligned with the first passage, and a bolt or screw extending through the first and second passages to secure the retaining ring to the carrier base.
 3. The carrier head of claim 2, wherein the bolt or screw extends through an aperture in the shim into the second passage in the retaining ring.
 4. The carrier head of claim 3, wherein the second passage does not extend into the lower portion of the retaining ring.
 5. The carrier head of claim 1, wherein the shim is composed of stainless steel.
 6. The carrier head of claim 1, wherein the first material is tube PPS and the second material is stainless steel.
 7. The carrier head of claim 1, wherein the seal and an outer portion of the flexible membrane are clamped between the upper portion of the retaining ring and the carrier base.
 8. The carrier head of claim 7, wherein the seal is clamped underneath the outer portion of the flexible membrane between the carrier base and the upper portion of the retaining ring.
 9. A method of using a retaining ring, comprising: securing the retaining ring to a base in a carrier head without a shim between the base and the retaining ring, an outer edge of a flexible membrane being clamped between the retaining ring and the base; polishing a first plurality of substrates with the carrier head, a lower surface of the retaining ring contacting a polishing surface; removing the retaining ring from the carrier head after the lower surface of the ring has been worn by a first amount; resecuring the retaining ring to the carrier head with a first shim between the base and the retaining ring to compensate for the wear of the lower surface of the retaining ring; and polishing a second plurality of substrates with the carrier head.
 10. The method of claim 9, further comprising: removing the retaining ring and first shim from the carrier head after the lower surface of the retaining ring has been worn by a second amount; resecuring the retaining ring to the carrier head with a second shim between the base and the retaining ring; and polishing a third plurality of substrates with the carrier head.
 11. The method of claim 10, further comprising securing a first seal between the flexible membrane and the retaining ring before polishing the second plurality of substrates, and securing a second seal between the flexible membrane and the retaining ring before polishing the third plurality of substrates.
 12. The carrier head of claim 10, wherein the second shim has a thickness that is greater than a thickness of the first shim by an amount that is substantially the same as the second amount.
 13. The carrier head of claim 10, wherein the second shim has a thickness that is sufficient to maintain proper function of the carrier head.
 14. The method of claim 10, further comprising securing a first seal between the flexible membrane and the retaining ring before polishing the second plurality of substrates.
 15. The carrier head of claim 9, wherein the first shim has a thickness that is substantially equal to the first amount.
 16. The method of claim 9, wherein the first shim has a thickness that is sufficient to maintain proper function of the carrier head.
 17. A kit, comprising: a retaining ring including a lower portion having a bottom surface for contacting a polishing pad during polishing and made of a first material, and an upper portion made of a second material which is more rigid than the first material, a plurality of channels formed in the bottom surface of the retaining ring for carrying slurry inwardly; a shim to be inserted between a base of a carrier head and the upper portion of the retaining ring; and an annular seal to be inserted between a flexible membrane and the retaining ring.
 18. A method of using a retaining ring, comprising: securing the retaining ring to a base in a carrier head with a first shim between the base and the retaining ring, an outer edge of a flexible membrane being clamped between the retaining ring and the base; polishing a first plurality of substrates with the carrier head, a lower surface of the retaining ring contacting a polishing surface; removing the retaining ring and first shim from the carrier head after the lower surface of the ring has been worn by a first amount; resecuring the retaining ring to the carrier head with a second shim between the base and the retaining ring; and polishing a second plurality of substrates with the carrier head.
 19. The method of claim 18, wherein the second shim has a thickness that is greater than a thickness of the first shim by an amount that is substantially the same as the first amount.
 20. The method of claim 18, wherein the second shim has a thickness that is sufficient to maintain proper function of the carrier head.
 21. A method of using a retaining ring, comprising: securing the retaining ring to a base in a carrier head without a shim between the base and the retaining ring, an outer edge of a flexible membrane being clamped between the retaining ring and the base; selecting a predetermined amount of wear for a lower surface of the retaining ring; polishing a first plurality of substrates with the carrier head, the lower surface of the retaining ring contacting a polishing surface; removing the retaining ring from the carrier head after the lower surface of the ring has been worn by the predetermined amount; resecuring the retaining ring to the carrier head with a first shim between the base and the retaining ring; and polishing a second plurality of substrates with the carrier head. 